Analytical Temperature Rising Elution Fractionation Research Articles

Fast successive self‐nucleation and annealing (SSA) thermal fractionation protocol for the characterization of polyolefin blends from mechanical recycling

AbstractThe sorting stage of mechanical recycling of post‐consumer polyolefins has severe challenges. Polypropylene (PP) is often contaminated with polyethylene (PE) and vice versa. To meet quality requirements, characterization of the recycled pellets is needed. To address this problem, fast characterization generating a statistical assessment of the content of the various batches from recycling is required. This investigation shows that the use of fast scanning rates (in a conventional Differential Scanning Calorimeter) in the successive self‐nucleation and annealing (SSA) protocol can reduce the thermal fractionation time, without losing resolution power, as long as the increase in heating/cooling rate is compensated by reducing sample mass. Using a “coupled SSA protocol” for polypropylene and polyethylene fractions at a rate of 10 °C/min, the measurement time is approximately 420 min. Implementing mass compensation, faster heating rates (i.e., 30 °C/min) and using a single‐fraction protocol, sufficient to determine the content of PP and high‐density PE, reduced the time of the measurement to 75 min. Examples of fractionations of commercial post‐consumer and post‐industrial recycled polyolefin blends conducted at a faster rate are provided. The derived polyolefin content is compared with the standard temperature rising elution fractionation analysis to assess the validity of the proposed method.

Engineered PP impact copolymers in a single reactor as efficient method for determining their structure and properties

Impact polypropylene (PP) copolymers (IPCs) are important materials for many commercial applications. These materials are usually synthetized through different methods involving two consecutive reactions in the same phase or in different phases. Here, a laboratory-scale synthesis method based on a sequential liquid- and gas-phase two-step process in a single reactor is developed. Propylene homopolymers and IPCs were synthesized with varying amounts of comonomers and hydrogen. The IPC materials obtained were fully characterized via analytical temperature rising elution fractionation (TREF), differential scanning calorimetry (DSC), 13C nuclear magnetic resonance (13C NMR), gel permeation chromatography with an infrared detector (GPC-IR5), Charpy impact, scanning electron microscopy (SEM), and cross-fractionation chromatography (CFC). The addition of only ethylene to the second step in the absence of hydrogen led to the creation of an ethylene-propylene (EP) copolymer with similar impact strength to that of a propylene homopolymer. The addition of hydrogen to the first step dramatically shortened the length of the PP chains and inhibited catalytic active centers that led to EP copolymer synthesis. This material exhibited very low molecular weight, low ethylene incorporation, and rubbery phases irregularly distributed along the isotactic polypropylene (iPP) matrix, resulting in the formation of an EP copolymer material with poor impact properties. IPCs synthesized without hydrogen and with a 50/50 (v/v) mixture of propylene/ethylene monomers in the second step enhance ethylene incorporation, facilitating adequate homogeneous and heterogeneous ethylene distribution and resulting in a high increment of amorphous ethylene-propylene-rubber (EPR) domains, which remarkably improves impact properties. Additionally, a criterion based on the ratio between EEE and EPE + PEP triads ranging between 1 and 2 was also established to predict the impact resistance of any heterophasic PP. Fractionation of the optimal sample provided a detailed understanding about the microstructure of this copolymer through the study of the molecular weight and composition of the fractions via GPC-IR, analytical TREF, and DSC measurements. Finally, the liquid–gas-phase two-step IPC material was compared, by means of SEM and CFC measurements, with synthesized IPC using liquid–liquid-phase two-step polymerization, and the results showed that the range of EP composition as well as ethylene distribution in the molar mass molecules of the IPCs was correlated to their mechanical behavior. This proves that crystalline families composed of high-molecular-weight EP copolymers in the liquid–gas-phase process can act as a compatibilizing agent between the iPP matrix and the elastomeric rubbery phase, allowing one to improve the impact resistance of the IPC, more so than that of IPCs obtained in the gas–gas and liquid–liquid phases. The results indicate that the synthesis of IPC resins in a single reactor is an efficient experimental method for fundamental research on IPCs.

Quantification of PP contamination in recycled PE by TREF analysis for improved the quality and circularity of plastics

Polyethylene constitutes more than one-third of the world's plastic and is one of the main components in municipal waste streams. Unfortunately, varying amounts of polypropylene (PP) are usually found in recycled high-density polyethylene (rHDPE), which erodes their mechanical properties and reduces the quality of these recyclates and their possible reuse in the industry. Differential Scanning Calorimetry (DSC) is the technique commonly used to detect and determine the presence of PP in these blends, but intrinsic limitations of this method lead to inaccurate results. Thus, alternative techniques must be employed to solve these drawbacks and help to estimate more accurately the levels of the PP impurities in rHDPE. In this work, crystallization techniques, such as Temperature Rising Elution Fractionation (TREF) is used to fractionate PE and PP blends into their components to obtain quantitative information. Municipal waste samples of rHDPE contaminated with different amounts of PP have been characterized by DSC and TREF. Comparative results of both techniques show an underestimation of PP values when DSC is used as analytical technique. Results prove that TREF analysis irrespective of the crystallization rate used is valid for the identification and quantification of PP impurities in rHDPE samples, even at low PP percentages in the blend lower than 2 wt. %, which could help to control the quality of rHDPE and achieve the priority of reusing and recycling adopted by the European Union for a Circular Economy of Plastics.

Polyethylene characterization by analytical temperature rising elution fractionation with evaporative light scattering detector

The temperature rising elution fractionation (TREF) analysis of linear and low-density polyethylene (PE) samples was investigated using an evaporative light scattering detector. Being less sensitive to pressure and temperature variations than the differential refractive index (DRI) detector, the ELSD allows the simplification of the flow path and reduction of the column dimensions. First experiments were done by quenching and capturing the injected polymer solution into a low volume TREF column. The elution profiles during the heating step showed that for each type of PE, clear correlations are found between the peak elution temperature and the PE density. Apart from having a signal to noise about 10 times better than DRI detection, the main advantage of the ELSD detector is the absence of the solvent peak. Because the solvent peak is evaporated by the ELSD, the initial 30 min of analysis can be used for dynamic cooling (meaning that the cooling step is done under solvent flow). Using dynamic cooling, we further develop the method to solve the challenging co-crystallization problem, when a mixture of low density and high-density PE is analyzed by TREF. By selecting specific dynamic cooling programs, it is possible to acquire in 30 min a baseline separation of two compounds and accurate peak elution temperatures, similar with those provided by classical TREF with a low cooling rate, performed over 11 h.

Characterization of the chemical composition distribution of polyolefin plastomers/elastomers (ethylene/1-octene copolymers) and comparison to theoretical predictions

Ethylene/1-olefin copolymers with a 1-olefin content >10 wt%, known as polyolefin plastomers (POP) and elastomers (POE), are materials of growing commercial importance. A relevant example are ethylene/1-octene (EO) copolymers. The thermal properties of nine 1-octene based POP/POE were studied by differential scanning calorimetry (DSC). The obtained thermograms were indicative of an asymmetric chemical composition distribution (CCD), triggering further investigations. At first, analytical temperature rising elution fractionation (a-TREF) was employed which gave further evidence of an asymmetric CCD for some samples. Given the low crystallinity of many of the samples, their CCD could, however, only be studied by high temperature high performance liquid chromatography (HT-HPLC). The hypothesis about an asymmetric CCD could be confirmed for all samples. The results were compared with theoretically calculated CCD (Stockmayer distributions) which are symmetrical. In a semiquantitative comparison, the main peak of the experimental CCD was found to be in good agreement with Stockmayer distributions.

Influence on properties and phase structure of single gas-phase reactor made impact polypropylene copolymers

Impact polypropylene copolymers (IPC) are materials important for many commercial applications. These materials are usually synthetized by different methods involving two consecutive reactions: in liquid, gas or liquid-gas phase. This work presents a method of synthesis in a laboratory scale based on two sequential steps in only one reactor in gas phase. Variables such as H2 addition, reaction time, and composition during the second step were studied, and their influence on the formation of propylene-ethylene copolymer materials with different properties was analyzed. The IPC materials so obtained were characterized by analytical temperature rising elution fractionation (TREF), calorimetric methods (DSC), 13C nuclear magnetic resonance (13C NMR), gel permeation chromatography with an infrared detector (GPC-IR5), Charpy impact, and scanning electron microscopy (SEM).The isotactic polypropylene matrix (iPP) obtained in the first step of the gas-phase process displays spherical morphology and lower mean particle size than those obtained in the liquid-phase process, even though the polymer grains experimented a certain tendency to agglomerate.In particular, hydrogen addition caused a significant decline in the catalyst productivity and dramatically shortened the length of the propylene homopolymer chains. Similarly, the presence of hydrogen on the synthesis of ethylene-propylene copolymers was demonstrated to lead to materials with very low molecular weight, low ethylene incorporation, and rubbery phases irregularly distributed along the iPP matrix and therefore with poor impact properties.On the other hand, ethylene-propylene copolymers synthesized without hydrogen yielded a suitable combination of molecular weights and molecular weight distribution that can contribute to good polymer processing and were proven to incorporate adequate amounts of ethylene mainly randomly distributed into the iPP matrix. SEM measurements revealed that the amorphous rubbery phase was homogeneously dispersed in the iPP matrix, and the Charpy test allowed to rank these materials as IPC.Finally, fractionation of IPC materials by preparative TREF provided information about the microstructure formation. Subsequent studying of molecular weights and composition of the fractions by GPC-IR, analytical TREF, and DSC measurements based on the ethylene-propylene composition and ethylene distribution in the molar mass molecules was shown to enable the design of IPC materials. Additionally, the optimal IPC material was compared with the best IPC using liquid-phase polymerization, and the results showed that the range of ethylene-propylene composition, as well as the ethylene distribution in IPC molar mass molecules, correlated with IPC mechanical behavior.

Analytical temperature rising elution fractionation of different polyethylene types using a column filled with carbon nanotubes

A high temperature gel permeation chromatograph (GPC) was coupled with a gas chromatograph (GC) oven to obtain an analytical temperature rising elution fractionation (TREF) system with evaporative light scattering detection. On this instrument, a new column partially filled with pristine carbon nanotubes (CNT) was tested by evaluating the elution profiles in function of temperature (thermograms) of different polyethylene (PE) types. By comparing these thermograms with those obtained with a traditional TREF column filled with metallic wires, the adsorption of polymer chains on the pristine CNT was clearly evidenced. The thermograms given by the column filled with CNT are similar with those provided by literature when chromatographic columns filled with porous graphitic carbon are used for this application, usually described as high temperature thermal gradient interaction chromatography (HT-TGIC).

Porous Organic Polymers-Supported Metallocene Catalysts for Ethylene/1-Hexene Copolymerization

Porous organic polymers (POPs) have received much attention in adsorption, separation, and catalysis. In this paper, porous organic polymers with different pore structure were used as metallocene catalyst supports, and ethylene/1-hexene copolymerizations were conducted using the POPs-supported metallocene catalyst. The pore structure of the prepared POPs and the supported metallocene catalyst were characterized by nitrogen sorption porosimetry and non-local density functional theory simulation, and the molecular chain structure of the produced ethylene/1-hexene copolymers were investigated through gel permeation chromatography (GPC), IR analysis, differential scanning calorimetry (DSC), and temperature rising elution fractionation (TREF). The results show that the loading amount of active sites varied with different pore structures of the POP supports, and the active species scattered in different pore sizes had a moderate impact on the molecular chain growth and the molecular weight distribution. The IR, DSC, and TREF analysis revealedthat different branching degree, double bond content, and chemical composition distributions were detected from the molecular chain structure of the ethylene/α-olefin copolymers from different POPs and silica-supported metallocene catalysts, despite their similar IR, DSC, and TREF curves due to the same active species. Scanning electron microscopy (SEM) showed that porous ethylene/α-olefin copolymers with varied surface morphology were obtained from the POPs-supported metallocene catalysts with different pore structure.

New method of single liquid-phase reactor synthesis of high-impact polypropylene: Structure, morphology, and impact properties of copolymers

High-impact polypropylene (hiPP) copolymer is a very important material in the industry because of its various applications. It is obtained in an industrial process involving two consecutive reactions either in the same or different phases. Unlike industrial processes that yield materials with a fixed amount of ethylene without the possibility of modifying the operation variables, we are presenting a new method to synthesize hiPP materials on a laboratory scale using two sequential steps in only one reactor. This approach permits the study of the influence of variables such as H2 addition, reaction time, and composition during the second step on the formation of propylene-ethylene copolymer materials with different properties. In the first step, the polypropylene matrix is synthesized in a liquid phase with or without addition of H2, and, in the second step, the elastomeric phase is also obtained in a liquid phase.The influence of the addition of hydrogen and ethylene and other operation conditions on the polypropylene matrix and the elastomeric phase distribution was studied and correlated to the mechanical and thermal properties of the material. Obtained materials were characterized by analytical temperature rising elution fractionation (TREF), calorimetric methods (DSC), 13C nuclear magnetic resonance (13C NMR), gel permeation chromatography (GPC), melt flow index (MFI), and scanning electron microscopy (SEM).The hydrogen addition remarkably enhanced the propylene polymerization while causing a significant decline in the ethylene polymerization rate. Based on these results, it seems that hydrogen has a complex effect on the catalyst behavior beyond its role as chain transfer agent. Furthermore, the presence of hydrogen in the polymerization caused a significant isotactic polypropylene (iPP) matrix molecular weight decay, which induced a modification of the viscosity ratio between the matrix and the rubbery phase produced in the second step. This yielded ethylene-propylene rubber (EPR) domains with higher average sizes and affected the phases’ morphology, resulting in a remarkable copolymer impact strength decline.The addition of ethylene to the second step in the absence of hydrogen affords an ethylene-propylene copolymer with high resistance to impact, which ranks it as a hiPP copolymer. Fractionation of this sample and the analysis of the obtained fractions provided knowledge about the microstructure of the copolymer and proved adequate distribution of the elastomeric phase into the iPP matrix.

Ternary Solid–Liquid Equilibria of Semicrystalline, Branched Polymer Solvent Mixtures—A Theoretical Study by Means of Lattice Cluster Theory

Analytical polymer separation techniques, like CRYSTAF (crystallization analysis fractionation) and TREF (temperature rising elution fractionation), play a crucial role in gaining insight into the interrelation between molecular architecture and properties. Often polymer samples are semicrystalline, as it consists of amorphous regions which exhibit a liquid-like structure, and ordered crystallized structures called crystallites. Additionally, polymers exhibit a certain degree of short-chain branching, and they are polydisperse with respect to molecular weight and chemical composition. To explain the theoretical background of common polymer separation techniques, a fair prediction of solid–liquid equilibria (SLE) of polymer solvent systems is required. By application of Lattice Cluster Theory (LCT), developed by Freed and co-workers, the molecular architecture can be considered. Recently, LCT was applied to calculate the SLE of binary semicrystalline, high-polymer solvent mixtures having an arbitrary molec...

Correlation between GPC, DSC, and Analytical TREF for Linear Polyethylene

Analytical temperature rising elution fractionation (TREF) of linear polyethylene (PE) samples with different densities was done in 1-chloronaphthalene using a gel permeation chromatograph (GPC) coupled with a gas chromatograph. The corrected peak elution temperatures completed the previously obtained data in trichlorobenzene, xylene, and dibutoxymethane. A mathematical correlation was found for diluted linear PE samples between the α parameter of the Mark-Houwink-Sakurada equation governing the retention time in GPC, the bulk melting temperature measured by differential scanning calorimetry (DSC), and the TREF peak elution temperature. The extrapolation to the melting temperature measured by DSC gives α = 0.5, thus confirming the hypothesis that polymer conformations in the melt are similar to those in a theta solvent.

Fast Analytical Temperature Rising Elution Fractionation (TREF) of Polypropylenes

Analytical temperature rising elution fractionation (TREF) is a complementary technique of gel permeation chromatography (GPC) for the analysis of polyolefin structure. By connecting a high-temperature GPC with a gas chromatograph (GC) oven it is possible to build a fast analytical TREF, which permits a dramatic reduction in analysis time by directly injecting the polymer solution onto the cold column, as compared with the traditional TREF in which the slow cooling step usually takes more than 40 h. The method was successfully applied on six commercial random and homo polypropylenes for which similar thermograms were obtained for fast- and slow-cooling TREF. The obtained results show subtle differences in the behavior of polypropylene melting in solution as compared with previously analyzed polyethylene. The most important is the difference of 12.6°C between the melting temperatures in the presence of trichlorobenzene and xylene, which is much higher for polypropylene than the 3.7°C measured for polyethylene.

Characterization of Polyethylene Using Fast Analytical Temperature Rising Elution Fractionation with Triple Detection (ATREF 3D)

A commercially available gel permeation chromatograph with triple detection (GPC3D) was coupled with a gas chromatograph to obtain a fast analytical temperature rising fractionation system with triple detection (ATREF3D). On this hybrid instrument we developed a fast method for evaluating in one run, and with less than 0.5 mg of sample, the density, molecular weight, and the type of polyethylene. The method was evaluated with excellent results on commercial polyethylene resins typically used for film and packaging applications.

Molecular structure and local dynamic in impact polypropylene copolymers studied by preparative TREF, solid state NMR spectroscopy, and SFM microscopy

The phase morphology in impact polypropylene was studied by size exclusion chromatography (SEC), differential scanning calorimetry (DSC), and solid-state nuclear magnetic resonance (NMR). The characterizations of the raw copolymer materials indicate the presence of amorphous and crystalline phases. In order to gain more comprehensive insight in the microstructure of these materials, four different subsamples were prepared by Temperature Rising Elution Fractionation (TREF) and subsequently analyzed by DSC, SEC, analytical TREF and solid state NMR measurements. Fraction 1 consisted of non-crystalline ethylene–propylene random copolymer (EPR). Fraction 2 was composed of intermediate ethylene and propylene segments with different length and capabilities to crystallize. In fraction 3, long propylene segments were the dominating contribution, although some amorphous and crystalline ethylene sequences were detected, which are probably incorporated into the polypropylene main chains, since no soluble component was detected by analytical TREF. Finally, isotactic polypropylene (iPP) was determined to be the unique component of fraction 4. The local molecular mobility in the different phases was analyzed using 2D Wide Line Separation (WISE) NMR, while 13C detected spin diffusion experiments helped to elucidate local morphologies. Scanning Force Microscopy (SFM) studies of the impact polypropylene revealed a semicrystalline iPP matrix with dispersed softer regions, which involve not only soft amorphous elastomeric EPR components, but additional nano-domains with semi-crystalline properties different from the iPP matrix. Combining the SFM and solid state NMR results, a phase model for the EPR regions dispersed in the iPP matrix is proposed.

Molecular heterogeneity of ethylene-propylene rubbers: New insights through advanced crystallization-based and chromatographic techniques

For a long time ethylene-propylene rubber (EPR) copolymers with high comonomer contents were believed to be amorphous materials with a random copolymer composition. This is not completely correct as has been shown by temperature rising elution fractionation (TREF) combined with differential scanning calorimetry (DSC), crystallization analysis fractionation (CRYSTAF), and high temperature–high-performance liquid chromatography (HT-HPLC). When using only conventional crystallization-based fractionation methods, the comprehensive compositional analysis of EPR copolymers was impossible due to the fact that large fractions of these copolymers do not crystallize under CRYSTAF conditions. In the present work, HT-HPLC was used for the separation of the EPR copolymers according to their ethylene and propylene distributions along the polymer chains. These investigations showed the existence of long ethylene sequences in the bulk samples which was further confirmed by DSC. The results on the bulk samples prompted us to conduct preparative fractionations of EPR copolymers having varying ethylene contents using TREF. Surprisingly, significant amounts of crystallizing materials were obtained that were analyzed using a multistep protocol. CRYSTAF and DSC analyses of the TREF fractions revealed the presence of components with large crystallizable sequences that had not been detected by the bulk samples analyses. HT-HPLC provided a comprehensive separation and characterization of both the amorphous and the crystalline TREF fractions. The TREF fractions eluting at higher temperatures showed the presence of ethylene-rich copolymers and PE homopolymer. In order to obtain additional structural information on the separated fractions, HT-HPLC was coupled to Fourier transform-infrared (FT-IR) spectroscopy. The FT-IR data confirmed that the TREF fractions were separated according to the ethylene contents of the eluted samples. Preparative TREF analysis together with a combination of various analytical methods proved to be useful tools in understanding the complex molecular composition of these rubber samples. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 863–874

Progress on Fast Analytical Temperature Rising Elution Fractionation (ATREF) Technique for Practical Applications

Fast analytical temperature rising elution fractionation (ATREF) is a recently proposed method to evaluate the molecular structure of polyolefins with unimodal distributions. In this article, we compare the results obtained by this method and the classical slow cooling rate ATREF. An interpretation of previous results found in the literature is proposed in order to explain one of the limitations usually associated with this technique. Practical recommendations are given to build columns adapted to this technique and to efficiently adjust the method parameters.

Fast Analytical Temperature Rising Elution Fractionation (ATREF) Method

Conventional analytical temperature rising elution fractionation (ATREF) is performed using slowly crystallized polymers in about 16 h. In this work, we developed a fast ATREF method in which the polymer sample is directly injected on the column at room temperature, thus reducing the analysis time to about 1 h. The method was tested using four metallocene polyethylenes with unimodal short chain branching distributions and different densities, previously analyzed by ATREF using a cooling rate of 0.1°C/min. The obtained results demonstrate that the fast ATREF method is very effective and accurate in evaluating short chain branching distribution for polyolefins having unimodal distributions.

Elution Temperatures of Polyethylene Copolymers in Temperature Rising Elution Fractionation (TREF): Comparison in Xylene and Trichlorobenzene Diluents

Temperature rising elution fractionation (TREF) became the preferred technique to characterize the short chain branching distribution of polyethylene copolymers. Due to technical limitations, preparative TREF (PTREF) is usually done in xylene, while trichlorobenzene is used in analytical TREF (ATREF). Attempts to correlate the TREF elution temperatures based on data published by different authors erroneously showed higher elution temperatures for xylene than for trichlorobenzene. Our study rectifies this error. The experiments were done in both solvents on the same analytical TREF instrument. For the analyzed polyethylene copolymers, we found that the average elution temperature in xylene is 3.7° ± 1°C lower than in trichlorobenzene.

Chemical compositional distribution of ethylene-1-butene copolymer prepared with heterogeneous ziegler-natta catalyst: TREF and crystaf analysis

Ethylene-1-butene copolymers were prepared with SiO2-supported TiCl4 catalyst by changing of 1-butene/ethylene molar ratio in feed, and the resulting copolymers were analyzed using temperature rising elution fractionation (TREF) and crystallization fractionation (Crystaf) methods to investigate the influence of C4/C2 molar ratio in feed on chemical compositional distribution and other parameters such as molecular weight and its distribution. TREF analysis showed that the copolymers had a broad and bimodal chemical compositional distribution (CCD) regardless of the content of 1-butene in the copolymer. The chemical composition was in the range of 5 to 55 branches per 1,000 carbons for all copolymers prepared in the study. Furthermore, the broader CCD was revealed for the copolymers having the higher content of 1-butene. Crystaf analysis did not showed a bimodal CCD for the copolymers having the 1-butene content of less than 5.1 wt%. The lower crystalline part having higher 1-butene content in Crystaf analysis was less than that of TREF analysis.

Block Index for Characterizing Olefin Block Copolymers

AbstractSummary: Olefin block copolymers produced by chain shuttling catalysis exhibit crystallinity characteristics that are distinct from what would be expected for typical random olefin copolymers with comparable monomer compositions produced from either ‘single‐site’ or heterogeneous catalysis. Olefin block copolymers produced by chain shuttling catalysis have a statistical multiblock architecture. A unique structural feature of olefin‐based block copolymers is that the intra‐chain distribution of comonomer is segmented (statistically non‐random). Fractionating an olefin block copolymer by preparative temperature rising elution fractionation, TREF, results in fractions that have much higher comonomer content than comparable fractions of a random copolymer collected at an equivalent TREF elution temperature. We have developed a “block index” methodology which quantifies the deviation from the expected monomer composition versus the analytical temperature rising elution fractionation, ATREF, elution temperature. When interpreted properly, this index indicates the degree to which the intra‐chain comonomer distribution is segmented or blocked. The unique crystallization behavior of block copolymers determine the magnitude of the block index values because the highly crystalline segments along an otherwise non‐crystalline chain tend to dominate the ATREF (and DSC) temperature distributions.